1. Field of the Invention
The present invention relates to a driving circuit and a driving method for a display device having plural electrodes such as a liquid crystal display device, and to a display device using the same.
2. Description of Related Art
Along with recent development of advanced visual and information society and widespread of a multi-media system, flat display devices such as a liquid crystal display device have become more and more important. The liquid crystal display devices have been widely used as display devices for portable devices because of advantages of low power consumption, slimness, and lightweight.
The liquid crystal display device includes a liquid crystal panel for displaying an image, and a driving circuit for driving the liquid crystal panel. An active-matrix type liquid crystal panel includes a device substrate, a counter substrate, and a liquid crystal filled in between the two substrates. Scanning lines are formed in a horizontal direction on the device substrate, and data lines are formed in a vertical direction thereon, and pixel electrodes are formed between the scanning lines and the data lines. Active elements such as TFTs (Thin Film Transistors) are formed around intersections between the scanning lines and the data lines. Gate electrodes of the TFTs are connected with scanning lines, source electrodes are connected with data lines, and drain electrodes are connected with the pixel electrodes.
The scanning lines are connected with a scanning line driving circuit, and the data lines are connected with a data line driving circuit. When the scanning line driving circuit sequentially drives the scanning lines, an output voltage of the data line driving circuit is applied to pixel electrodes through the TFTs. On the other hand, a common electrode opposite to the pixel electrode is formed on the counter substrate. The common electrode is applied with an appropriate level of voltage by a common electrode driving circuit. As a result, a voltage corresponding to a potential difference between the pixel electrode and the common electrode is applied to the liquid crystal. The liquid crystal display device changes a voltage level applied to the liquid crystal to change the orientation of liquid crystal grains and transmittance and thus produces gray scales.
In general, a polarity of a voltage applied to pixel electrode from the data line through the TFT (hereinafter referred to as “pixel voltage”) is inverted at predetermined intervals in order to prevent deterioration of displayed image quality. To that end, for example, an AC driving method such as a dot-inversion driving method that changes a polarity of the pixel voltage from one pixel to another has been adopted. With such an AC driving method, the pixel electrode is alternately applied with a positive voltage and a negative voltage, so a large amount of power is consumed. As a countermeasure against this, there has disclosed a technique of setting a voltage of a data line to an intermediate level at the time of inverting a polarity to thereby save power consumption (see Japanese Patent Translation Publication No. 9-504389 and Japanese Unexamined Patent Application Publication No. 11-95729, for instance).
Japanese Patent Translation Publication No. 9-504389 discloses a driving circuit where a data line is short-circuited to a common node by a multiplexer upon the polarity inversion, and a potential of the data line is kept at an intermediate level (common electrode potential) by an external storage capacitor connected with the common node. Further, this publication also discloses a driving circuit that dispenses with the external storage capacitor and connects all data lines to a common node by a multiplexer to set the potential of each data line to about an intermediate level in the case of line-inversion driving.
Japanese Unexamined Patent Application Publication No. 11-95729 discloses a driving circuit where adjacent data lines are temporarily short-circuited upon the polarity inversion and their potentials balance each other out and average out to around the intermediate level. Further, this publication also discloses a driving circuit where all data lines are short-circuited and connected with a common electrode voltage source to keep the potentials of all the data lines to the intermediate level.
However, the above driving circuits have the following problems. That is, in the driving circuit as disclosed in Japanese Patent Translation Publication No. 9-504389, a voltage of the data line is set to an intermediate level upon the polarity inversion by means of the external capacitor. This leads to a problem in that an additional external part is required and costs high. Meanwhile, if the external capacitor is not provided, there are on-resistances corresponding to two switches on a path where the data lines are short-circuited, so a time constant increases and it takes much time to set a voltage of the data line to around the intermediate level. As a result, it is impossible to efficiently set the voltage of the data line to the intermediate level, leading to an increase in power consumption. In the case of short-circuiting the data lines, heat is generated from the on-resistances corresponding to the two switches.
Meanwhile, in the driving circuit as disclosed in Japanese Unexamined Patent Application Publication No. 11-95729, a pair of adjacent data lines is short-circuited upon the polarity inversion to average the voltage levels to about the intermediate level. Thus, an average voltage is determined depending on a pixel voltage of a pair of adjacent data lines. Accordingly, an average voltage may vary from one pair of adjacent data lines to another. It is feared that display characteristics deteriorate like variations of the brightness. Further, although an ideal average voltage is a common electrode voltage, in the case of averaging voltage values every data line pair as mentioned above, an average voltage varies depending on individual voltage values of the adjacent data lines, so an average voltage is not always around the common electrode voltage. If the average voltage deviates from the common electrode voltage, a voltage range of a writing voltage is increased, and power consumption increases. This leads to heat generation in the worst case. If adjacent ones of all the data lines are short-circuited through switches and connected with the common electrode voltage source, as many on-resistances as the switches are used, resulting in a problem in that a time constant increases and it takes much time to set the voltage of the data line to the intermediate level.